3. Sea-ice ecosystems and climate change

The sea ice in the Arctic and parts of the Antarctic is disappearing rapidly. The ice loss is especially marked in the Arctic. The decline in Arctic sea-ice extent now affects nearly all seasons, peaking in summer. Furthermore, in the past four decades the Arctic sea ice has become thinner as a result of global warming. Older ice, which has survived numerous summers, is shrinking rapidly and being replaced by seasonal ice, which completely disappears each year in spring and summer. In addition, refreezing starts later in the year. This means that overall there is a longer ice-free period. The snow layer is becoming thinner and experts expect that, by the end of the century, rain could be the dominant form of precipitation in the Arctic. This would make the ice much darker, and the reflection of shortwave radiation from the sun (the so-called albedo) would decrease, causing the ice to absorb more heat and to melt more rapidly. All in all, in the future the Arctic sea ice will be thinner, younger and shorter-lived than ever before. In the Antarctic, too, for several decades, researchers have recorded sea-ice loss in a number of regions – especially in the western Antarctic in the vicinity of the Antarctic Archipelago (IPCC, 2019).

Declining sea ice in both polar regions means that the habitat of those plants and animals that rely on the ice as a source of food, for protection or as a nursery ground will become increasingly scarce. In the Arctic, the ice is retreating ever farther northward between spring and autumn. At the same time, the water temperature is rising. The habitat of organisms that have adapted to the cold or that live directly on the sea ice is constantly shrinking. In the future, they will have to retreat to the last remaining ice zones. Animals that mainly hunt for food at the ice edge, such as whales and seabirds, will be forced to migrate ever farther north to follow the ice. At the same time, species from more southerly latitudes are migrating to the warmer polar waters that are becoming ‘vacant’. In this respect, it is feared that in the future, stocks of several specialised polar plant and animal species will shrink, while species from the Atlantic and Pacific will penetrate the region (WOR, 2019). Experts speak of the ‘Atlantification’ and ‘Pacification’ of the Arctic. These migrant species may compete with the native species for food. We now also know that microalgae from the Atlantic sector are far less energy-rich than the ice algae. As such, the decline of the sea ice and the loss of sea algae are a threat to the survival of those animals that feed on the ice algae, like small crustaceans and the fishes that hunt them. In winter 2017/2018, for example, the ice cover off the coast of Alaska was only half the size it was in 1978, when satellite measurements of the ice cover began. The low ice cover meant that the algal bloom in spring 2018 was very small. As a result, little food was available for small crustaceans and other zooplankton. Since important Arctic fish species like capelin and polar cod feed on zooplankton, ultimately their stocks also declined. In the following summer, an abnormal number of seabird deaths were observed in Alaska, which were attributed to the lack of fish. This example makes it clear how fundamentally sea-ice loss affects the polar food web (WOR, 2019).

Researchers still don’t completely understand the food cycles and dynamics of the ice-algae populations, since numerous factors play a role – the water temperature, ocean currents, the temperature and salt content of the water, and the availability of food. One aspect that is currently being intensively investigated is how thinning sea ice and larger areas of open water affect the situation. However, we do know for certain that a total loss of sea ice in summer would have far-reaching consequences.

Whereas the sea-ice decline in the Arctic is progressing steadily, the situation in the Antarctic is not as clear. While for decades, the sea ice in parts of the western Antarctic has been decreasing, for a long time the ice in the eastern Antarctic appeared to be increasing. However, the ice extent also seems to have been decreasing here in recent years. In the Antarctic, experts have focused mainly on the relationship between the ice algae, krill and those animals that feed on krill – penguins, as well as several seal and whale species. If krill stocks decline as a result of climate change, all predator species could be affected. In the western Antarctic, there is already evidence of decreasing krill stocks. Current research is investigating just how serious the situation there is.